1. Field of the Invention
The present invention relates to a technical area of depth measurements of a guiding instrument, and more particularly to a multi-point calibration method for the depth of a horizontal directional drilling guiding instrument.
2. Description of the Related Art
At an early stage, horizontal guiding instruments apply a dipole source and two antennas to measure the depth of a probe.R=d/((s1/s2)^1/3−1)
where, s1 and s2 are intensities of signals transmitted from a transmitter and received respectively by two detecting instruments having a one-dimensional antenna, and d is the distance between the two detecting instruments having the one-dimensional antenna. This depth measuring method usually has a relatively large depth error.
Another present existing method is a single-point calibration method that obtains a measured value Mo, provided that the distance (k0) is known, and the depth (r) is calculated by the following equation:r=(M0/s)1/3 
where, s is the intensity of a signal transmitted from a transmitter and received by a measuring instrument, and this method provides a more accurate depth measurement, but the range of the accurate measurement is very narrow due to other factors such as the existence of electric fields and environmental noises. The accuracy will be increasingly lower after a certain specific depth is reached. The greater the distance, the greater is the error.